The present invention relates to a substrate for a photoelectric conversion device and a photoelectric conversion device using the same.
In a photoelectric conversion device, a transparent conductor on which a transparent conductive film of tin oxide, ITO, or the like is formed on a glass surface is used as a substrate. As the transparent conductive film, a tin oxide film has been used widely. A silicon-based thin film photoelectric conversion device in which a silicon-based material is used for a photovoltaic layer has been receiving attention due to the low energy cost required for its manufacture.
The transparent conductive film for the thin film photoelectric conversion device is required to have a high transmittance (i.e. to introduce a larger quantity of light into a photovoltaic layer) and a low sheet resistance (i.e. to reduce the loss in leading out generated electricity). It has been known that to provide the surface of the transparent conductive film with proper roughness is effective in trapping light in the photovoltaic layer.
As a preferable example of the shapes of convex or concave portions of such a rough surface, for example, JP 61-288473 A discloses the shapes of convex portions with heights in the range between about 100 nm and 500 nm and intervals between respective convex portions in the range between about 200 nm and 1000 nm. Further, JP 2-503615 A discloses shapes of convex portions having diameters in the range between 0.1 xcexcm and 0.3 xcexcm and the ratio of height/diameter in the range between 0.7 and 1.2. JP 4-133360 A discloses shapes of convex portions having truncated pyramidal or pyramidal shapes with heights in the range between 100 nm and 300 nm and angles between their ridgelines and the line perpendicular to the substrate in the range between 30 and 50 degrees.
On the other hand, when using a transparent conductive film having convex portions with excessively large diameters due to large crystal grain sizes in general, the conversion efficiency of a thin film photoelectric conversion device decreases. It is conceivable that the decrease in such a characteristic is affected by the deterioration in a step coverage property by a photovoltaic layer on the transparent conductive film, the increase in light absorption at the interface between layers due to the deterioration in film quality of the photovoltaic layer, or the like. In addition, the increase in resistance due to poor junction between the photovoltaic layer and the transparent conductive film also is conceivable as one of the factors causing the performance deterioration.
As a means for observing the shapes of convex or concave portions in the transparent conductive film, a scanning electron microscope is used in general. When simply the shapes of individual convex or concave portions or the intervals therebetween are to be measured, they can be observed sufficiently by a scanning electron microscope. However, when the scanning electron microscope is used, a high magnification is required and therefore the surface condition of the film is evaluated over a small region (a square with about 1 to 2 xcexcm side length) at any one time. It is difficult to obtain a precise distribution of the convex portions with large diameters from the evaluation over such a small region.
The present inventors confirmed that even in a transparent conductive film in which no convex portion with a large diameter was found by the observation over such a small region as described above, scattered convex portions with large diameters were found by an observation over a larger region in some cases. Such a surface condition also was confirmed in the transparent conductive films formed according to the methods described in the above-mentioned publications.
As described above, conventionally, no attention has been paid to providing the surface of a transparent conductive film with roughness capable of suppressing the deterioration in film quality of a photovoltaic layer while providing a light trapping effect.
The present invention is intended to provide a substrate for a photoelectric conversion device that enables the improvement in conversion efficiency by controlling the shapes of convex or concave portions of the surface of a transparent conductive film over a larger region than that in a conventional method. The present invention also is intended to provide a photoelectric conversion device using this substrate.
In order to achieve the aforementioned objects, a substrate for a photoelectric conversion device according to the present invention includes a glass sheet, an undercoating film formed on the glass sheet, and a transparent conductive film containing tin oxide as a main component formed on the undercoating film. The surface of the transparent conductive film has convex portions and concave portions. The convex portions have a mean diameter in a range between 0.05 xcexcm and 0.3 xcexcm and include five convex portions or less with diameters of at least 0.5 xcexcm per 100 xcexcm2 of the surface. In this specification, the xe2x80x9cmain componentxe2x80x9d denotes a component accounting for at least 50wt. % of the whole amount.
According to the above-mentioned configuration, a suitable substrate for a photoelectric conversion device (a photovoltaic device) such as a thin film solar cell or the like can be provided. The above-mentioned shape of the surface is effective in trapping light and the surface does not include many convex portions with large diameters. Therefore, the deterioration in film quality of a photoelectric conversion layer or the like is suppressed, thus obtaining an excellent conversion function.
The present invention also provides a photoelectric conversion device using the aforementioned substrate. In this photoelectric conversion device, at least one photoelectric conversion unit and a back electrode are stacked in this order on the transparent conductive film of the above-mentioned substrate. This photoelectric conversion device is used with the glass sheet side positioned as a light incident side.